Zinc ionomer rubber activator

ABSTRACT

Curable rubber compositions and cured articles are based on the use of zinc activator compositions for the sulfur cure. Activator compositions contain zinc and a polymeric component having a plurality of COOH groups, at least some of which are neutralized with the zinc. With polymeric zinc activators, sulfur cure can be activated at levels of zinc below those used in conventional systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 13/652,197 filed Oct. 15, 2012, which is adivisional application of U.S. patent application Ser. No. 12/628,743,filed Dec. 1, 2009, now U.S. Pat. No. 8,288,467, issued Oct. 16, 2012,claiming priority to U.S. Provisional Application 61/120,728, filed Dec.8, 2008, all of which are hereby incorporated herein by reference, eachin its entirety.

INTRODUCTION

Synthetic and natural rubbers have a variety of unique and usefulphysical properties. In an uncured or natural state, such materialsgenerally exhibit properties that are less than optimal for everyday orindustrial use. Accordingly, rubber compositions are generally reactedwith crosslinkers such as those containing sulfur or peroxide in orderto cure the rubber to produce industrial articles having acceptableproperties. In general, heat is applied to a rubber composition during amolding operation to produce molded articles having desired physicalproperties.

For many reasons, it is often desirable to increase the rate of cure insuch molding operations. If the cure rate can be increased, articles canbe molded for shorter times or they can be molded for the same time atlower temperatures. In either instance, a process is generally more costeffective if the rate of cure is faster. Over the years, a variety ofadditives has been developed that can be added to rubber compositions togenerally increase the cure rate. Examples of such additives include thewell-known sulfenamide accelerators. Using the known accelerators,rubber compositions can be formulated having a wide range of cure rates.In general, it would be desirable to provide rubber compositions havingeven greater cure rates so as to achieve the benefits noted above.

Zinc oxide (ZnO), as well as other divalent metal oxides such as CaO andMgO, is commonly referred to as an activator for the cure of rubbercompounds containing accelerators, especially those containing sulfur.More precisely, one molecule of cationic component (e.g. ZnO) combineswith two molecules of a fatty acid such as stearic acid (C₁₇H₃₅COOH) toform a salt or soap such as zinc stearate (Zn(C₁₇H₃₅COOH)₂) and water.The soap (e.g., the zinc stearate) is believed to be the activator forcure.

During mixing and/or cure, the ZnO (or other cationic component) andfatty acid react to form the soap, which activates the sulfur cure. Inactual practice, a stoichiometric excess of the cationic component tofatty acid is employed in the rubber compounds to obtain sufficientactivation.

The excess of cationic component such as ZnO in the cured rubber tendsto weaken the physical properties of the cured rubber. Additionally,zinc tends to leach out of cured rubber compositions over time, whichcould lead to concerns over contamination.

In light of the situation, it would be desirable to provide methods andcompositions for curing rubber that have low levels of zinc, yet whichstill give acceptable cure kinetics and physical properties in theresulting cured articles.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

Curable rubber compositions and cured articles made from thecompositions are provided based in part on the discovery of zincactivator compositions for the sulfur cure. In general, the activatorcompositions contain zinc and a polymeric component having a pluralityof COOH groups, at least some of which are neutralized with the zinc.Although the invention is not limited by theory, it is believed that thepolymeric zinc activators activate the sulfur cure by a mechanismsimilar to that of the classic zinc stearate activators known in theart. It has now been discovered that when polymeric zinc activatorsaccording to the invention are used, rubber cure by sulfur crosslinkingsystems can be activated using a total concentration of zinc that iswell below that used in conventional zinc stearate systems, especiallythose with any filler that has active hydrogens on its surface, e.g.˜Si—OH on the surface of silica fillers.

In one aspect, curable compositions contain polymeric activatorscontaining zinc neutralized carboxyl groups. The invention furtherprovides methods of curing the rubber compounds and the cured articlesmade from them. In other aspects, the curable compositions contain asource of zinc such as zinc oxide as well as carboxylic polymers thatcontain a plurality of COOH groups. The invention provides methods ofmixing the zinc and carboxylic polymer separately to form a curablerubber composition with good activation. It is believed that when thezinc and the carboxylic acid polymer are provided separately andpre-mixed in this way, an active ingredient like the partially zincneutralized carboxylic polymer is formed in situ. Once the activatingsystem is formed, either by direct addition to a composition containinga curable rubber or by in situ formation, the rubber compositions arefurther compounded and subjected to curing conditions.

A variety of rubber articles can be made using the zinc activationsystem described herein. The cured articles can contain a variety offillers, such as silica and carbon black. Silica-filled rubbers of theinvention find use, for example, as rubber soles for shoes, while filledcarbon black compositions can be used in tires.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show cure parameters of compositions of the invention.

DESCRIPTION

Curable rubber compositions contain zinc activator components. Theactivator components comprise a zinc part and a carboxylic polymer part.The carboxylic polymer is generally a polymeric material that contains aplurality of COOH groups, either on the backbone of the polymer orgrafted onto the backbone. In various embodiments, the activatorcomponent is described and presented as a polymer containing a pluralityof COOH groups, wherein at least a portion of the COOH groups areneutralized with at least one metal that can activate the cure of therubber resins. In other embodiments, the activator component isdescribed as one comprising a polymer with a plurality of zinccarboxylate groups. In other embodiments, the activator component ischaracterized as a zinc ionomer. As further discussed herein, the zincactivator component can be provided into a rubber composition in apre-formed fashion, or can be generated in situ from a zinc source and acarboxylic polymer source.

In various embodiments, the activator component comprises a second metalthat is compatible with and enhances the activation by the zinc.Preferably, these secondary metals are also divalent like zinc.Preferred metals include magnesium and calcium.

In various aspects, the invention provides moldable rubber compositions,methods for curing the molded rubber compositions, and cured articlesmade by the process of curing the compositions. Molded articles find useas rubber soles of shoes, as automotive tires, and in a wide variety ofother uses.

In one embodiment, a moldable rubber composition contains a rubberresin, a curing system for the rubber resin, a filler component, and anactivator. The rubber resin is selected from natural rubber andsynthetic rubber, while the activator comprises a polymer that containsa plurality of COOH groups, wherein at least a portion of the COOHgroups are neutralized with at least one metal that can activate cure ofthe rubber resin. In preferred embodiments, the at least one metal thatcan activate cure of the rubber resin includes zinc, particularly whenit is present as the Zn+2 ion. In various further embodiments, thecomposition further comprises calcium or magnesium.

In various embodiments described herein, the curing system for therubber resin comprises sulfur and the activator comprises a polymer witha plurality of COOH groups, wherein at least a portion of the COOHgroups are neutralized with zinc.

In this and other embodiments described herein, the filler component canbe selected from a variety of substances such as, for example, silicaand carbon black.

In another embodiment, a moldable rubber composition is characterized ascontaining a rubber resin, a sulfur-containing curing system for therubber, a filler, and an activator for sulfur cure of the rubber whereinthe activator comprises a polymer with a plurality of zinc carboxylategroups. As in other embodiments, the rubber resin can be selected fromnatural rubber and also from synthetic rubbers that can be sulfur cured.

In another embodiment, a moldable rubber composition is characterized bycontaining rubber, a sulfur cure system for the rubber, a filler, and anactivator component comprising a zinc ionomer. As used herein, a zincionomer is a term of art used to describe and characterize polymerscontaining carboxylate groups on the main chain or pendant from the mainchain, and wherein groups of carboxylate groups are associated with zincions in such a way as to form an ionic cluster. The ionic clusters actlike thermally labile crosslinks.

In other embodiments, methods of curing a rubber composition involveheating compositions described herein under time and temperatureconditions sufficient to provide a cured article. As described furtherherein, in various embodiments, moldable rubber compositions are curedfor a total time of T90, T90+1 minutes, T90+2 minutes, and so forth. Thetemperature of curing is selected so that activation and cure of therubber resin takes place in a commercially reasonable amount of time.

Molded articles made by curing the moldable rubber compositions of theinvention include tires, rubber soles for shoes, and the like.

In another aspect, a zinc activator component is formed in situ duringthe mixing of the other components of the moldable rubber compositions.Thus, in one embodiment, a rubber composition contains a natural orsynthetic rubber, a polymeric material containing a plurality of COOHgroups, a Zn⁺² compound, and a filler. In various embodiments of theinvention, such a rubber composition is first compounded, blended, ormasticated for a sufficient time and at a suitable temperature for thezinc compound to react with the polymer to form a zinc activatorcomponent in situ that comprises a polymeric material containing aplurality of zinc carboxylate groups. Before, while, or after theactivator complex is formed in situ by blending the above composition,one or all of the components of the sulfur-curing system for the rubberresin is added. Once the sulfur-curing system and the activator complexare present in the composition containing the curable rubber, theresulting composition is further blended for a suitable time, preferablyat a temperature at which the rubber does not significantly cure. Theresult of the process is a moldable rubber composition that can be madeinto rubber soles, tires and the like by molding the rubber compositionand subjecting it to curing conditions of time and temperature.

Thus, in various embodiments, a method of making a cured rubbercomposition involves the steps of heating a moldable rubber compositionfor a suitable time at a suitable temperature, wherein the compositioncomprises a rubber resin selected from natural rubber and syntheticrubber, a sulfur-curing system for the rubber, a rubber cure activatorcomponent comprising a polymer with one or more carboxylate groups, andfurther containing 0.01-0.5 wt. % Zn an elemental basis. The zinc isbelieved to present at least in part in close association with thecarboxylate groups. As already noted, the 0.01-0.5 wt. % Zn on anelemental basis can be provided in the compositions by in situ formationof a polymeric zinc activator, or by the use of commercial polymericmaterials containing zinc carboxylates as activator component.Advantageously, the compositions contain a lower total level of Zn thanconventional zinc stearate activated compositions.

In various other embodiments, methods of making a moldable rubbercomposition involve the steps of applying mechanical energy to a rubbercomposition to mix the components. In the first step where mechanicalenergy is applied, the rubber compositions contain in a non-limitingembodiment a natural or synthetic rubber resin, a polymer containing aplurality of COOH groups, a compound that is a source of Zn⁺² ion, and afiller. After the above components are mixed, a sulfur curative systemfor the rubber is added and the components mixed for a further time at atemperature below one at which the rubber cures significantly.Thereafter, the mixing is stopped and the mixture is allowed to cool.The cooled blend is then ready to be molded and cured to provide avariety of cured rubber articles as described further herein.

Further non-limiting description of components of the moldable rubbercompositions is now provided. Unless the context requires otherwise, itis understood that components described for illustrative purposes in oneembodiment can also be used in others.

Rubber Resins

The rubber compositions of the invention contain natural or syntheticrubber, or mixtures of rubbers, as well as conventional rubber additivessuch as curing agents and accelerators.

In general, any rubber that can be crosslinked by a sulfur cure can beused to make the compositions of the invention. Sulfur cured describesthe vulcanization process typical of making rubber. Mixtures of rubbersmay also be used. Examples of rubbers useful in the invention include,without limitation, natural rubber such as those based on polyisoprene.

Synthetic rubbers may also be used in the invention. Examples include,without limitation, synthetic polyisoprenes, polybutadienes,acrylonitrile butadiene rubber, styrene acrylonitrile butadiene rubber,polychloroprene rubber, styrene-butadiene copolymer rubber,isoprene-isobutylene copolymer rubber and its halogenated derivatives,ethylenepropylene-diene copolymer rubbers such asethylene-propylene-cyclopentadiene terpolymer,ethylene-propylene-5-ethylidene-norbornene terpolymer, andethylene-propylene-1,4-hexadiene terpolymer, butadiene-propylenecopolymer rubber, butadiene-ethylene copolymer rubber,butadiene-isoprene copolymer, polypentenamer, styrene-butadiene-styreneblock copolymers, epoxidized natural rubber and their mixtures. Ingeneral, such compounds are characterized by repeating olefinicunsaturation in the backbone of the polymer, which generally arises fromthe presence of butadiene or isoprene monomers in the polymer structure.

Sulfur Curing Agents

Conventional sulfur based curing agents may be used in the compositionsof the invention. Such curing agents are well known in the art andinclude elemental sulfur as well as a variety of organic sulfide,disulfide and polysulfide compounds. Examples include, withoutlimitation, vulcanizing agents such as morpholine disulfide,2-(4′-morpholinodithio)benzothiazole, and thiuram compounds such astetramethylthiuram disulfide, tetraethylthiuram disulfide anddipentamethylenethiuram tetrasulfide. The vulcanizing agents may be usedalone or in combination with each other. In a preferred embodiment,sulfur is used as the curing agent.

Accelerators

The rubber compositions of the invention also in general containaccelerators. Such accelerators and co-accelerators are known in the artand include without limitation, those based on dithiocarbamate,thiazole, amines, guanidines, xanthates, thioureas, thiurams,dithiophosphates, and sulfenamides. Non-limiting examples ofaccelerators include: zinc diisobutyldithiocarbamate, zinc salt of2-mercaptobenzothiazole, hexamethylenetetramine, 1,3diphenyl guanidine,zinc isopropyl xanthate, trimethyl thiourea, tetrabenzyl thiuramdisulfide, zinc-O—, O-di-n-butylphosphorodithiolate, andN-t-butyl-2-benzothiazole sulfenamide.

Another accelerator suitable for use is a class of xanthogenpolysulfides such as dialkyl xanthogen polysulfide. A non-limitingexample of a dialkyl xanthogen polysulfide is diisopropyl xanthogenpolysulfide, such as is commercially available as Robac AS-IOO, suppliedby Robac Chemicals. Advantageously, Robac AS-IOO is free of nitrogen,phosphorus, and metallic elements. It is recommended for use as anaccelerator in vulcanization of natural rubber, synthetic polyisoprene,nitrile rubber, etc. where the formation of N-nitrosamines and type-4allergens is of prime concern. The dialkyl xanthogen polysulfides alsoact as a sulfur donor.

In various embodiments, the dialkyl xanthogen polysulfide acceleratorsshow synergism with other accelerators, for example, zinc dibenzyldithiocarbamate or tetrabenzyl thiuram disulfide (TBzTD), whether in thepresence or absence of elemental sulfur. The xanthogen polysulfides alsoproduce effective cure in the presence of thiazoles and sulfenamides.For example, a combination of the xanthogen poly sulfide and asulfenamide, such as N-t-butyl2-benzothiazole sulfenamide (TBBS), is aneffective accelerator system that functions in the absence of elementalsulfur. The xanthogen polysulfide accelerators can also be used incombination with other soluble sulfur donors. Typical treatment levelswith the xanthogen polysulfide as accelerator are from 0.1-10 PHR, andare advantageously 5 PHR or less, or 2 PHR or less. In a non-limitingexample, a xanthogen polysulfide accelerator compound is used at about0.6 PHR.

The sulfur based curing agents and accelerators together make up asulfur curing system. Normally, both the curing agent (source of sulfur,including soluble and insoluble sulfur, and including organic andinorganic sulfur) and the accelerator should be present before carryingout the rubber curing reactions described herein.

In various embodiments, the rubber compositions of the invention containan accelerator selected from the class of titanium or zirconiumcompounds. Such accelerators are described in U.S. Pat. No. 6,620,871,the disclosure of which is incorporated by reference. The zirconium ortitanium compounds can be characterized as those that contain an alkoxygroup —OR bonded respectively to titanium or zirconium. Mixtures of thezirconium and titanium compounds may also be used. Generally, the Rgroup of the alkoxy group is an alkyl group having 8 or fewer carbonatoms. In a preferred embodiment, the R group contains 6 or fewercarbons, and more preferably contains 4 or fewer carbon atoms. Examplesof alkyl groups containing 4 or fewer carbon atoms include methyl,ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, and t-butyl.

In a preferred embodiment, the titanium or zirconium compound has 2alkoxy groups bonded to the titanium or zirconium. In another preferredembodiment, there are 4 alkoxy groups —OR bonded to the central titaniumor zirconium atom where R is as described above. Based on the above,there are several forms of the titanium and zirconium compounds.Preferred compounds include the tetraalkyl (those having four alkoxygroups bonded to the metal) and the chelate forms. A class of compoundsthat has shown good utility is the chelates. Chelates, in general, arethose titanium or zirconium compounds that are complexed with an organicligand system that contains two atoms or functional groups capable offorming covalent or dative bonds to the central titanium or zirconiumcompound. Generally, the atoms or functional groups that form covalentor dative bonds to the central atom are those that are highlyelectronegative and include oxygen, nitrogen, and sulfur. The two atomsor functional groups providing the ligand to the central titanium andzirconium atoms may be the same or different. In a preferred embodiment,the atoms bonded to the central titanium or zirconium atom through thechelate are oxygen. Examples of chelating ligands include, withoutlimitation, acetylacetonate, ethyl acetylacetonate, triethanolamine,lactic acid and its salts such as the ammonium salt, glycolic acid andits salts, and esters of citric acid, such as diethyl citrate. A wellknown chelate useful in the invention is the titanium acetylacetonatechelate compound illustrated by the following formula.

Where R comprises an alkyl group of 8 carbons or less, preferably 6carbons or less, and more preferably 4 carbons or less. Here the chelatecontains two alkoxy groups OR and a central titanium atom on which twoacetylacetonate groups are chelated. It can be seen that the titaniumatom, the two dative bonding groups on the chelate molecule, and theatoms bridging the two dative bonding groups form a six membered ring.In general, chelates of the invention include those that form from afive to an eight membered ring with the titanium atom and the two dativebonding groups of the chelating ligand. In the figure, the R groups areas described above. In a preferred embodiment, the R groups in thefigure are isopropyl. Other chelates may be used in the rubbercompositions of the invention. The above figure is provided forillustration only. In other preferred embodiments, other chelatingligands such as triethanolamine, lactic acid ammonium salt, diethylcitrate, and ethyl acetylacetonate are used. It is also possible tosubstitute zirconium for titanium in the structure illustrated above.

The zirconium or titanium chelates are generally highly colored, rangingfrom yellow to a dark red. This generally provides no problems if thechelate compounds are to be formulated into black rubber compositions.On the other hand, if white or lightly colored rubber compositions areto be formulated, then tetraalkyl and polymeric forms of the titaniumand zirconium compounds are preferred, as they are not as deeplycolored.

In various embodiments, the compositions contain an effective amount ofthe titanium or zirconium compound. Generally, the compositions willhave from about 0.01 parts to about 10 parts per hundred parts of rubberresin (phr) of the titanium or zirconium compound. Depending on thevariety of rubber, and additives such as accelerators and fillers,formulations can be compounded at amounts ranging from about 0.1 toabout 5 phr by weight of the titanium or zirconium compound. Generally,the compounds are available in as supplied form from about 70% togreater than 98% active.

In a preferred embodiment, the titanium or zirconium compounds have fouralkoxy groups. For a titanium compound, the general structure can berepresented by the formula

where the structure depicted is a tetraalkyl titanate. For a zirconiumcompound, the formula is the same except that zirconium is substitutedfor titanium as the central atom. The organic side chains can berepresented by R1, R2, R3, and R4. In general, R1, R2, R3, and R4 can bethe same or different. When the R groups are identical it is common todepict the tetraalkyl titanate by the general formula Ti(OR)4. Anexample of a tetra alkyl titanate where all the R groups are identicalis tetra n-butyl titanate. In the formula, the titanium has 4 alkoxygroups OR wherein R is an alkyl group of 8 carbons or less. Preferably Ris an alkyl group of 6 carbons or less. In a preferred embodiment, thealkyl groups are of 4 carbons or less as discussed above for thechelates. It appears that the length of the pendant organic groupdetermines the effectiveness of the titanium or zirconium compound toreduce the cure temperature of rubber compositions or the invention.Without being bound by theory, it is believed that the lower molecularweight side chains such as those containing 8 carbons or less andpreferably 6 carbons or less and more preferably 4 carbons or less, willcause the titanium or zirconium compound to break down at relativelylower temperatures and hence produce their catalytic effect. As themolecular weight of the pendant organic groups increases, the compoundin generally becomes more thermally stable and is less capable of havinga catalytic effect on cure.

Another sub-class of titanium or zirconium compounds is the polymerictitanates or zirconates. The titanates can be represented by the generalstructure

where x represents the degree of polymerization. The zirconates are likethe titanates, with zirconium substituted for titanium. Such polymerictitanate materials can be made by condensing a tetraalkyl titanate, asdescribed for example in U.S. Pat. No. 2,621,193. The analogouspolymeric zirconates can be synthesized by the procedure outlined forthe polymeric titanates. The R group in the alkoxy group —OR in theformula above is defined as for the chelates and for the tetraalkyltitanates.

Titanium and zirconium compounds are commercially available for examplefrom DuPont under the Tyzor® trade name. One example is Tyzor AA-75,which is a 75% solution in isopropanol of the titanium diisopropylacetylacetonate given in Table 1. Another is Tyzor® BTP, which is polyn-butyl titanate.

It may be desirable to protect the titanium and/or zirconium compoundsfrom hydrolysis during use. In general, it is observed that thetetraalkyl zirconates and titanates tend to have a higher rate ofhydrolysis than the chelate compounds. The susceptibility to hydrolysisincreases as the size of the R group on the alkoxy group —OR decreases.The zirconium compounds tend to be more sensitive than the titaniumcompounds to moisture. When susceptibility to hydrolysis of thecompounds of the invention is a concern, it has been found useful toprovide the zirconium or titanium compounds of the invention asauxiliary compositions in the form of masterbatches. To make themasterbatches, the zirconium or titanium compound is admixed with ahydrophobic material that protects it from moisture. In a preferredembodiment, the hydrophobic compound comprises a petroleum wax. When thezirconium compound or titanium compound is provided in liquid form, itis often desirable to add a carrier to the composition to bind thezirconium or titanium compound. A commonly used carrier is silica. Whendark colored or black rubber compositions are to be formulated it ispossible to use carbon black as a carrier. When light colored or whiteformulas are to be made, it is possible to use titanium dioxide as acarrier to make the masterbatches of the invention. As an example, amasterbatch was formulated from the liquid tetra n-propyl zirconate bythe following procedure. First, 30 grams of silica were weighed out. Thesilica was heated at 175° C. for 20 minutes and allowed to cool in anoven to less than 100° C. The silica lost 1.8 grams of weight duringheating due to evaporation of water. Next, Okerin 1956 was heated tomelting. Okerin 1956 is a petroleum wax. A 70% solution of tetran-propyl zirconate in n-propanol, 63 g, was added to the silica andstirred to make a paste. Then 39 grams of the molten Okerin 1956petroleum wax was stirred onto the silica paste and stirred to combine.The mixture was allowed to cool slightly and sealed in a plastic bag.The masterbatch uses a petroleum wax, Okerin 1956, to protect thezirconate from moisture. The silica acts as a convenient carrier to bindthe liquid zirconate. The zirconate masterbatch was used in severalformulations and compared to the previous studies where only the liquidform was used. In principle, it was observed that the masterbatch didprotect the zirconate from decomposition by water. It was observed thatthe cure time of a rubber composition containing as a component thezirconate masterbatch was reduced at a specific temperature whencompared to the rubber composition made by using the liquid zirconate.

Rubber Compounding

The rubber compositions of the invention can be compounded inconventional rubber processing equipment. In a typical procedure, allcomponents of the rubber composition are weighed out. The rubber andadditives are then compounded in a conventional mixer such as a Banburymixer. If desired, the compounded rubber may then be further mixed on aroller mill. At this time, it is possible to add pigments such as carbonblack. The composition may be allowed to mature for a period of hoursprior to the addition of sulfur and accelerators, or they may be addedimmediately on the roller mill. It has been found to be advantageous toadd the accelerators into the Banbury mixer in the later stages of themixing cycle. Adding the accelerators into the Banbury mixer generallyimproves their distribution in the rubber composition, and aids in thereduction of the cure time and temperatures that is observed in thecompositions of the invention. In general, the elemental sulfur curingcompound is not added into the Banbury mixer. Organic sulfides (sulfurdonating compounds) may be added to the Banbury mixer.

Rubber Cure Activators

[The activator component, in various embodiments, contains a carboxylicfunctional base copolymer (also referred to as a “carboxylic polymer”)and zinc. It is believed that, in the activator composition, the zinc isincorporated into the base polymer by being associated with thecarboxylic acid groups of the polymer such as by neutralization. Asnoted, there are several zinc incorporated carboxylic-based polymersthat are commercially available. These commercially availablezinc-neutralized polymers can be incorporated as the activatorcomponent. Alternatively, a carboxylic base polymer and a source of zinccan be combined as described herein to form the activator complex insitu.

In one embodiment, the base copolymer is a polymer of an a-olefin and anα,β-ethylenically unsaturated carboxylic acid. The α-olefin has ageneral formula RCH═CH₂, where R is a radical selected from hydrogen andalkyl radicals having from 1-8 carbon atoms. The α,β-ethylenicallyunsaturated carboxylic acid has 1 or 2 carboxylic acid groups and isalso referred to as a carboxylic acid monomer. In various embodiments,the olefin content of the polymer is at least 50 mol %, while the acidmonomer content of the polymer is generally from about 0.2-50 mol % andis 0.2-25 mol % in various embodiments. In the active component, thequantity of zinc metal ion is generally sufficient to neutralize atleast about 10 percent of the carboxylic acid groups in the basepolymer. In some embodiments, it is preferred to avoid completeneutralization of all the carboxylic groups by zinc.

Suitable olefins for forming the base copolymer include ethylene,propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 3-α,β-ethylenicallyunsaturated carboxylic group containing monomers include those having3-8 carbon atoms. Examples include acrylic acid, methacrylic acid, ethylacrylic acid, itaconic acid, maleic acid, fumaric acid, as well asmonoesters of dicarboxylic acids, such as methyl hydrogen maleate,methyl hydrogen fumarate, ethyl hydrogen fumarate, maleic anhydride, andso on. For purposes of the description of the base copolymers, maleicanhydride is considered to be a carboxylic acid group containing monomerbecause its chemical reactivity is that of carboxylic acid groups.Similarly, other α,β-ethylenically unsaturated anhydrides of carboxylicacids can be used. The concentration of the acidic monomer and copolymeris from 0.2% mol-50% mol, preferably from about 1-10% mol %.

The base copolymers can be prepared in a number of ways. In one example,the copolymers can be obtained by copolymerizing a mixture of the olefinand the carboxylic acid monomer. In other embodiments, copolymers ofa-olefins and α,β-ethylenically unsaturated carboxylic acid can beprepared by copolymerizing the olefin with an unsaturated carboxylicacid derivative, which subsequently or during copolymerization isreacted completely or in part to form the free acid. Thus, hydrolysis,saponification, or pyrolysis can be employed to form an acid copolymer(i.e., a carboxylic functional base copolymer) from an ester copolymer.

Other monomers besides the olefin monomer and carboxylic acid monomercan be used to form the base copolymer. Non-limiting examples of basecopolymers include: Ethylene/acrylic acid copolymers,ethylene/methacrylic acid copolymers, ethylene/itaconic acid copolymers,ethylene/methyl hydrogen maleate copolymers, ethylene/maleic acidcopolymers, ethylene/acrylic acid/methyl methacrylate copolymers,ethylene/methacrylic acid/ethyl acrylate copolymers, ethylene/itaconicacid/methyl methacrylate copolymers, ethylene/methyl hydrogenmaleate/ethyl acrylate copolymers, ethylene/methacrylic acid/vinylacetate copolymers, ethylene/acrylic acid/vinyl alcohol copolymers,ethylene/propylene/acrylic acid copolymers, ethylene/styrene/acrylicacid copolymers, ethylene/methacrylic acid/acrylonitrile copolymers,ethylene/fumaric acid/vinyl methyl ether copolymers, ethylene/vinylchloride/acrylic acid copolymers, ethylene/vinylidene chloride/acrylicacid copolymers, ethylene/vinyl fluoride/methacrylic acid copolymers,ethylene/chlorotrifluoroethylene/methacrylic acid copolymers andcarboxylated rubbers such as carboxylated acrylonitrile-butadiene rubber(XNBR) and hydrogenated carboxylated acrylonitrile-butadiene rubber(HXNBR).

The zinc-containing polymeric activator components of the presentinvention are obtained by the reaction of the copolymer base with anionizable zinc compound. The reaction is referred to as neutralization,and the activator components are described as containing a plurality ofcarboxylic groups neutralized by zinc. As noted, some zinc-containingactivator components are available commercially, while others can beprepared in situ from a zinc compound and a carboxylic functional basecopolymer. Formation of the zinc-neutralized carboxylate groups duringin situ production of the zinc activator complex can be followed, forexample, with infrared spectroscopy. Activator components are selectedthat contain a suitable level of zinc in a carrier material comprisingthe carboxylic functional base copolymer. Normally, very high levels ofzinc neutralization are avoided because the neutralized polymer tends tobecome intractable. On the other hand, when too small a percentage ofthe acid (i.e., carboxylic) groups is neutralized, the component doesnot provide sufficient zinc for activation. It has been found desirableto provide zinc activator components wherein about 5-50 percent of theacid groups have been neutralized. In keeping with the environmentalgoal to reduce zinc levels, one can preferably neutralize 5-20 percentof the acid groups.

Commercially available zinc activator components based on olefincarboxylic acid copolymers include those available under trade namessuch as Surlyn™ (DuPont), Amplify™ (Dow) and Iotek™ (ExxonMobil).

Other examples of an olefin carboxylic copolymer are terpolymers such asthose available under the trade name Lotader™ from Arkema. Anon-limiting example is a terpolymer of ethylene, an acrylic ester, andmaleic anhydride.

In addition to carboxylic copolymers described above containingcarboxylic groups pendant from the main chain, other suitable carboxylicpolymers include those prepared by grafting maleic anhydride onto othercopolymers. Suitable examples of these include those available under thetrade name Orevac™ from Arkema.

Another class of base copolymers that can be combined with a zinccomponent to provide zinc carboxylate activator complex is the class ofstyrene maleic anhydride copolymers. A family of styrene maleicanhydride copolymers is commercially available, for example, fromSartomer, in the SMA™ series. In various embodiments, suitable baseresins are commercially available that contain styrene/maleic anhydrideratios of 1:1, 2:1, 3:1, 4:1, 6:1, and 8:1. Ester resins are prepared bypartial esterification of the base resins. The ester resins contain acombination of anhydride and monoester/monocarboxylic acidfunctionality. The commercial grades are conveniently available in flakeor powder form. The number average molecular weight of variouscommercially available styrene-maleic anhydride copolymers and partialesters runs from about 2,000-12,000. An example of a partiallyesterified styrene maleic anhydride polymer is Sartomer SMA 3840. It isbelieved to be partially esterified with isooctanol.

The styrene maleic anhydride copolymers are available as acarboxylic-containing base polymer. In preferred embodiments,zinc-containing activator components are formed in situ by combining thestyrene-maleic anhydride copolymers and partial esters with a zinccompound such as ZnO according to the methods described herein.

In addition to zinc, the rubber compositions can also contain otherdivalent metals that have been found to enhance the cure, sometimes in asynergistic fashion with the zinc. In various embodiments, thecompositions further contain magnesium or calcium. Although not limitedto theory, it is believed that rubber compositions containing Mg or Cain addition to zinc contain the respective Mg and Ca in a form closelyassociated with carboxylic groups of the base copolymers, so as to formadditional carboxylate groups on the polymers. The Mg- or Ca-containingactivators can be formed in situ by combining suitable compounds such asMgO and CaO along with the zinc compounds in the methods describedherein. The divalent Ca or Mg is generally added to the compositionsherein as a compound or compounds comprising Ca²⁺ or Mg²⁺. Althoughpotentially any such compound can be used, it is preferred to use therespective hydroxides (Ca(OH)₂ or Mg(OH)₂) or oxides (CaO or MgO).

Fillers

Fillers are used in rubber compositions to enhance properties, to savemoney, to facilitate processing, to improve physical properties or forother reasons. A variety of filler materials are known. Such fillersinclude silica, carbon black, clay, organic fiber, inorganic metalpowder, mineral powder, talc, calcium sulfate, calcium silicate, and thelike. Typical levels of these and other fillers include from about 10phr to 100 phr or higher. In various embodiments, the compositionscontain 10-80, 30-70, 40-60, 50-60, or 35-60 phr filler.

Silica is preferred in some embodiments. In preferred embodiments, thefiller comprises a silica filler in an amount such as 10 to 100 phr. Infurther non-limiting embodiments, the compounds comprise 30 to 60 phrsilica filler or 40 to 60 phr silica filler. Without limitation, typicalcompositions for use in preparing molded rubber outsoles for athleticshoes contain about 10 to about 60 phr filler.

Coupling Agents

The rubber compositions may also contain coupling agents, such as thosebased on silanes. When present, the silane coupling agents contribute tothe stability and physical properties of the compositions, for example,by compatibilizing or coupling the reinforcing filler with the rubbercomponents. Silane coupling agents include those with amino, epoxy,(meth)acryl, chloro, and vinyl functionality.

Examples of amino functional silane coupling agents includeaminopropyltriethoxysilane; aminopropyltrimethoxysilane;aminopropylmethyldimethoxysilane; aminoethylaminopropyltrimethoxysilane;aminoethylaminopropyltriethoxysilane;aminoethylaminopropylmethyldimethoxysilane;diethylenetriaminopropyltrimethoxysilane;diethylenetriaminopropyltriethoxysilane;diethylenetriaminopropylmethyldimethoxysilane;diethylenetriaminopropylmethyldiethoxysilane;cyclohexylaminopropyltrimethoxysilane;hexanediaminomethyldiethoxysilane; anilinomethyltrimethoxysilane;anilinomethyltriethoxysilane; diethylaminomethyltriethoxysilane;(diethylaminoethyl)methyldiethoxysilane; andmethylaminopropyltrimethoxysilane.

Examples of sulfur functional silane coupling agents includebis(triethoxysilylpropyl)tetrasulfide;bis(triethoxysilylpropyl)disulfide; bis(3-ethoxydimethylsilylpropyl)oligosulfur; mercaptopropyltrimethoxysilane;mercaptopropyltriethoxysilane; mercaptopropylmethyldimethoxysilane;3thiocyanatopropyltriethoxysilane; and bis(silatranylpropyl)polysulfide.

Examples of epoxy silane coupling agents include:glycidoxypropyltrimethoxysilane; glycidoxypropyltriethoxysilane;glycidoxypropylmethyldiethoxysilane; andglycidoxypropylmethyldimethoxysilane.

Examples of (meth)acryl silane coupling agents include:methacryloxypropyltrimethoxysilane; methacryloxypropyltriethoxysilane;and methacryloxypropylmethyldimethoxysilane.

Examples of chloro silane coupling agents include:chloropropyltrimethoxysilane; chloropropyltriethoxysilane;chloromethyltriethoxysilane; chloromethyltrimethoxysilane; anddichloromethyltriethoxysilane.

Examples of vinylyl silane coupling agents include:vinyltrimethoxysilane; vinyltriethoxysilane; andvinyltris(2-methoxyethoxy)silane.

Moldable rubber compositions contain other ingredients in addition tothe rubbers, curatives, accelerators, and activators. These additivesare well-known in the art and include processing aids, antioxidantpackages, pigments, and the like. Non-limiting examples of specific usesof these additives are given in the examples.

Using the zinc ionomer activators described herein, it has been foundthat moldable rubber compositions can be cured to provide useful moldedarticles, even when they contain much less zinc than would be requiredfor conventional zinc stearate activated systems. In variousembodiments, the use of zinc in the moldable rubber compositions is atan order of magnitude less than that used with conventional zinc soapactivation. The reduced level of zinc in the moldable rubbercompositions and cured articles is believed to lead to a variety ofadvantages. Moldable rubber compositions with a lower level of zinc fromuse of the inventive zinc activators are cheaper than conventionalrubbers even taking into account the extra expense of the basecopolymers on which the zinc activators are based.

Because the compositions and cured articles contain less overall zinc,there is less leaching of zinc into the environment. It is believed thatcarboxylate groups of the base copolymers are tightly bound to the zincions and further limit the leaching of the zinc from the compositions.In various embodiments, this is home out in measurements of total zincand of so-called “leachable” zinc.

In a non-limiting example, total Zn analysis is carried out digestingthe rubber following EPA Method 30S0 (5-g sample, nitric acid+peroxide)and then analyzing for Zn by ICP (SW601O). The ICP method is inductivelycoupled plasma atomic emission spectroscopy. Typical cured rubbercompositions contain from about 0.01 to about 05% by weight of zincdetermined in this way, reflecting a significant reduction of zinclevels over conventional rubber: more detail on zinc level is givenbelow.

Leaching analysis for Zn is carried out following EPA method 1312 (SPLP,using pH=4.2 extraction fluid option) to leach the sample and thenanalyzing the leachate by ICP (SW6010) for Zn. Cured rubber compositionsdescribed herein are characterized by low levels of leached zinc, asreflected in the leaching analysis and ICP concentration determination.In various embodiments, the measured zinc leachate levels are <50 μg/L,<40 μg/L, <30 μg/L, <20 μg/L, <10 μg/L, or <5 μg/L according to the EPAmethods (including ICP analysis of the leachate). Although the leachatelevels are low, they are non-zero. Normally, the leachate zinc levelswill be above 0.1 μg/L, above 0.5 μg/L, or above 1 μg/L. In contrast,conventional cured rubbers activated by zinc stearate exhibit leachatelevels above 50 μg/L, or around 75 μg/L.

Furthermore, it is observed that leachable zinc is reduced by a greaterpercentage than is total zinc, when comparing the current ionomeractivated cured compositions to conventional zinc activatedcompositions. Accordingly, in various embodiments the invention providesa method of decreasing the amount of leachable zinc in proportion to thetotal zinc in a rubber cured by zinc activation of a sulfur based curingsystem, by curing a zinc containing rubber composition in the presenceof an activator containing a base copolymer containing a plurality ofCOOH groups, wherein the activator can be zinc containing and isdescribed herein. In an example, the leachable zinc was 74.2 μg/L(micrograms Zn per liter) for a standard zinc stearate activated rubbercompound with 15,800 mg/kg of zinc. For the same compound where theactivator is based on an ionomer with zinc, the concentration of zinc is2880 mg/kg. Using proportions, one would expect that the leachable zincwould be ca.13.5 μg/L for the composition with lower zinc. But themeasured leachate zinc was only 4.43 μg/L, or about ⅓ of that “expected”on the basis of the total zinc content. In various embodiments, it isobserved that the measured value of leachable zinc is reduced by 50% ormore (but not all the way to 100%) compared to that expected or“predicted” based on just a reduction in the overall zinc level in thecured rubber. This is consistent with a model where the zinc is moretightly bound in the cured rubber activated with an ionomer than it isin a conventional rubber activated with a zinc fatty acid soap.

In various embodiments, it is found that the shelf life of compoundedrubbers using the zinc activators described herein often increases, sothat the compounds can be held for longer periods of time before curing.Advantageously, it is found that physical properties of molded articleswith the lower levels of zinc are equivalent to conventionally curedrubber compounds. In various embodiments, it is even observed that thetear strength can be improved. Although the invention is not limited bytheory, the improvement in tear strength may be due to the differentforms in which zinc is present in the compounded rubbers.Conventionally, ZnO powder is added in large excess to the availablestearic acid. Some ZnO powder particles will not react with the stearicacid and remain in the cured rubber compound. These particles can act asstress concentration points and facilitate crack propagation; i.e.tearing. In the present invention, the zinc often is only present in amolecular form as part of an ionomer. In the molecular form, it does notaffect crack propagation.

Even though much lower levels of zinc are used in the compositionsdescribed herein, it can be observed that activation is improved whencompared to a conventional zinc stearate activator. The improvedactivation is reflected in a lower activation energy measured duringcure of the inventive compounds. The lower activation energy means thateither less time or a lower temperature is required to effect cure ofthe compositions.

Although the invention is not limited to theory, it is believed that theactive zinc activator complexes described herein sequester zinc orotherwise prevent it from participating in side reactions on the surfaceof various rubber components such as filler. To illustrate, a typicalrubber formula may contain 100 parts of rubber, 0-100 PHR (PHR=parts per100 parts of rubber) filler, 5 PHR ZnO, 1 PHR stearic acid, 0-25 PHRsulfur, and various other ingredients—accelerators, retarders, processoil, anti-oxidants, and so forth. Of note is that the mass ratio of ZnOto stearic acid is generally high, as much as about 5:1. Assume 5 gramsof ZnO to one gram of stearic acid and using their respective formulaweights, it is easy to calculate the molar excess of ZnO (see Table Ibelow, where Zn(StA)₂ represents zinc stearate).

TABLE I Calculation of ZnO to stearic acid molar excess ComponentsFormula Wt (g/mo1e) ZnO 81.4 Stearic Acid 285.4 Zn(StA)₂ 632.3 ZnOmmoles Mmoles mmoles Excess PHR ZnO StA ZnO required mmoles ZnO 5.0061.43 3.50 1.75 59.67

In conventional zinc stearate activation, the Zn is believed to bedepleted in a side reaction. Zn stearate is believed to be able to reactwith hydroxyl groups on the surface of a filler, such as precipitatedsilica. At the first stage of the reaction, one stearic acid molecule isgenerated and Zn binds to one surface hydroxyl. Provided there is moreZnO, another stearic acid molecule is liberated, and the Zn then bindsto two surface oxygen atoms.

The cessation of the above reaction scheme upon depletion of ZnO hasbeen shown via FTIR, where the species with Zn bound to only one stearicacid and a surface hydroxyl is only resolved via FTIR as theconcentration of Zn stearate approaches zero. This suggests that thereaction to deplete Zn and regenerate stearic acid is very fast. Toavoid any issues with insufficient activation of the cure, ZnO istraditionally added in large stoichiometric excess to stearic acid.Thus, in conventional zinc stearate activation, a large molar excess ofzinc to stearic acid is required to overcome this step of contaminationof the filler, which essentially depletes zinc for use in activation.

Regarding the zinc activator complexes described herein, it is believedthat the carboxylate groups on the carboxylic based copolymers sequesterthe zinc and protect it from such reaction with surface hydroxyls of thefiller. For this reason, the zinc is not depleted during blending andcuring of rubber compositions and can be provided at much lower levels.As seen in the example, conventional zinc stearate is used incompositions where the ZnO concentration is on the order of 3 to 5 PHR.As further demonstrated in the examples, suitable rubber compositionsand cured articles can be made within the equivalent concentration ofZnO in the compositions is 0.25 PHR or lower. At the lowest levels, thisequates to more than a 90% reduction in zinc in the compositions. Whilea typical sulfur cured rubber contains on the order of 1.6 to 4% or 1.6to 3.2% or about 1.7 to 2.3% elemental zinc, compositions of theinvention containing the zinc ionomer activators can typically containabout 0.01 to 0.5% or about 0.04 to 0.4% elemental zinc, in anon-limiting example, where all percentages of elemental zinc arepercent by weight of the cured composition.

In contrast to conventional curing, when ZnO is provided in molar excessrelative to the stearic acid to provide useful zinc soaps foractivation, the zinc activator components of the invention actuallycontain a deficit of Zn+2 to carboxyl groups. Despite this molar deficitof Zn⁺², the reaction on the silica surface cannot be observed by FTIR.While the invention is not limited by any theory, it is proposed thatthe zinc may be tightly bound with the carboxylic groups. In somepolymer systems, the zinc carboxylates are believed to form ionicclusters similar to the zinc ionomers, such as the Surlyn® products. Inmaleic anhydride copolymers or graft copolymers, a maleic anhydride ringopens to form two carboxylic acids, which react with ZnO to form ionicclusters similar to ionomers.

Curing of the moldable rubber compositions to provide molded articles iscarried out by conventional means. Normally, a moldable rubbercomposition is introduced into a mold, where it is subjected toconditions of curing temperature and pressure for a time sufficient todevelop the properties of the cured article. As is conventional, thecuring is characterized by parameters Ts2 and T90. These parameters aremeasured according to known methods by measuring the torque of anoscillated rubber disk while it is being cured. The time for developmentof 90% of the final torque is referred to as T90. The parameter Ts2 is ameasure of how fast the cure develops and is called a measure of“scorch.” In various embodiments, cured rubber compositions arecharacterized by cure under conditions where cure proceeds for a timeequivalent to T90 or to T90 plus one minute. In this way, a cured rubberis cured to a stage at which, if measured by a rheometer, at least 90%of the total increase in torque from the uncured to the fully curedstated occurred or would have occurred due to the curing conditionsimposed thereon.

It is observed that with moldable rubber compositions containing thezinc ionomer activators described herein, cure is activated equivalentlyor better than conventional zinc stearate activators, even at lower andsometimes much lower relative contents of Zn. A measure of the improvedcure can be demonstrated by calculating an activation energy for thesystem. An activation energy is calculated according to conventionalmethods by measuring a cure parameter at different temperatures andcalculating the activation energy from an Arrhenius plot. It isconvenient to use Ts2 as the cure parameter in calculating theactivation energy. For a typical ZnO/stearic acid activated system, theactivation energy E_(act) is measured to be about 89 kJ/mol.

Because of the improved cure as reflected in the calculations ofactivation energy, a lower temperature can be used to effect the cure,thus saving energy, or time can be saved at standard cure temperature.

Shaped articles made by curing the moldable rubber compositions take anumber of forms such as shoe outsoles, tires, and the like. The curedarticles are characterized by containing a sulfur crosslinked rubberresin (detectable by for example infrared spectroscopy and measurementof elastomeric properties of the cured article) and in addition apolymer with a plurality of zinc carboxylate groups. Presence of thelatter can be confirmed or detected with spectroscopic methods and/orelemental analyses. Advantageously, the cured articles have sufficientlycured rubber, but contain less zinc than is conventional for zincstearate activated rubbers. In various embodiments, the elemental zinclevel is 0.01-1% by weight or 0.01 to 0.5% by weight. In variousembodiments, the shaped articles are characterized by total zinc contentof 0.05-1% or 0.05-0.5% or 0.05-0.3%, with all percentages by weight. Instill other embodiments, the total zinc content is about 0.1-1%,0.1-0.5%, or 0.1-0.3% by weight of elemental zinc. Elemental zinc in themoldable compositions and cured articles can be detected and quantifiedby conventional methods such as atomic absorption spectroscopy and x-rayfluorescence.

The shaped articles are believed to contain the polymer with a pluralityof zinc carboxylate groups by virtue of the activator complex beingincluded in the moldable rubber composition from which the cured articleis made. That is, the copolymer component of the activator does notundergo significant chemical transformation during cure, and the zincpresent in the starting moldable rubber composition remains in theshaped articles after cure. Since the action of the activator isbelieved to be at least in part catalytic, i.e. participating in thereaction but not being consumed or changed, the zinc carboxylate groupsof the activator complex are believed to be present in the shapedarticles. Such groups can be detected by infrared spectroscopy.

It is well known that Zn(C₁₈H₃₅0₂)₂ (zinc stearate) has a strong peak at1536 cm⁻¹ in the IR spectrum due to asymmetric stretching of CO₂. Otherlow molecular weight zinc esters, such as zinc dimethacrylate have thissame peak. For the ionomers described herein, this peak is not apparent.

In neat ionomers partially neutralized with zinc, there is a peak at1466 cm⁻¹. For the compounded rubber samples of the present invention,this peak is resolved around 1468-1471 cm⁻¹. In some embodiments whenthe ionomer is made in situ, this peak is not seen in the uncuredrubber, but is observed in the cured rubber, thus indicating its in situformation. Another interesting observation is that in the partiallyneutralized ionomers, the zinc can be partially extracted from the ionicclusters by the addition of stearic acid to the compound. Partialextraction is evidenced by peaks at both 1536 cm⁻¹ (zinc stearate) and1470 cm⁻¹ (zinc ionomer). In the above sample containing stearic acid,there was a small peak at 1596 cm⁻¹ which is evidence of reaction ofzinc stearate at the silica surface.

EXAMPLES

The following examples will show how a partially neutralized ethylenemethacrylic acid polymer activates cure when the neutralizing ion iszinc.

The following compounds are listed in examples.

In the tables below Surlyn 9910 is a terpolymer of ethylene, acrylicester, and maleic anhydride, partially neutralized with zinc, availablefrom DuPont.

Surlyn 8940 is an ionomer partially neutralized with sodium availablefrom DuPont.

NBR-Styrene is an acrylonitrile-butadiene-styrene terpolymer rubber.

BF is butadiene rubber.

BHT is butylated hydroxyl toluene.

PEG is polyethylene glycol.

Silane is a silane coupling agent.

Example 1

TABLE 1 Example 1A 1B 1C 1D 1E 1F Component PHR PHR PHR PHR PHRComponent PHR Surlyn 9910 0 2 5 10 20 Surlyn 8940 20 NBR-Styrene 6.256.125 5.9375 5.625 5 NBR-Styrene 5 BR 93.75 91.875 89.0625 84.375 75 BR75 Process aid 1 1 1 1 1 Process aid 1 Black pigment 2 2 2 2 2 Blackpigment 2 Silica 50 50 50 50 50 Silica 50 Soybean-oil 2 2 2 2 2Soybean-oil 2 Silane 0.5 0.5 0.5 0.5 0.5 Silane 0.5 BHT 1 1 1 1 1 BHT 1Wax 0.5 0.5 0.5 0.5 0.5 Wax 0.5 PEG 2.5 2.5 2.5 2.5 2.5 PEG 2.5 Processaid 1 1 1 1 1 Process aid 1 <<<<<CURATIVE PACKAGE>>>>> TBzTD 0.21 0.210.21 0.21 0.21 TBzTD 0.21 MBTS 0.6 0.6 0.6 0.6 0.6 MBTS 0.6 Sulfur 2.212.21 2.21 2.21 2.21 Sulfur 2.21 (below is for information and is not anadditional ingredient to the formulas) Percent elemental 0.0 0.04 0.100.19 0.39 0.0 zinc

Approximately 3 kg batches of the formulae given in Table 1 were mixedin a laboratory kneader. The compound was allowed to cool for at leastfour hours prior to addition of a curative package (TBzTD, MBTS,sulfur/SU135) on an open mill. The compound is allowed to “mature” forone day, then is pressed into slabs for physical testing. The typicalpressing conditions are 120 to 220° C. for a time equivalent to a cureof >T90 on a Rheometer. This means that >90% of the increase in torquedue to crosslinking of the rubber has occurred. The pressing processcreates a cured rubber.

One test to determine if the cure has been activated is a rheometer.This test measures the change in torque of a sample as it cures at aspecified temperature. This equates to a cure time. For example, once90% of the increase in torque has occurred (T90), then the cure time canbe set for the temperature of the test. For the present studies, thecure time is T90+1 minutes. Another time that is recorded is Ts2. Thisis referred to as the time to “scorch”. It indicates the onset of curingreactions.

If the sample is insufficiently activated, then the cure time will belonger. In FIG. 1, the cure times (Ts2 and T90) for the above examples1A˜1F are depicted. As can be seen, when there is no Surlyn 9910(Example A), then the cure does not activate well, and T90 is >12minutes. With just 2 PHR of Surlyn 9910 (Example B) the cure is wellactivated, and T90 is <3.5 minutes. As the Surlyn 9910 content increasesto 20 PHR, the cure is still well activated with T90-5.1 minutes. By wayof comparison, typical cure times for an analogous formula activatedwith 3.5 PHR ZnO (equal to 1.7% elemental zinc based on the totalformula weight) and 1PHR stearic acid are: Ts2, 1.33-1.88 minutes andT90, 4.32-5.38 minutes. It appears that the Surlyn 9910 is less“scorchy” (longer Ts2) while still activating the cure in a reasonabletime. Being less “scorchy” is a positive attribute, since it makesprocessing easier, and indicates an increased shelf life for thecompounded rubber.

If the Surlyn 8940 resin is used, T90 appears similar to the compoundwithout any Surlyn. This is quite easily explained, as the Surlyn 9910is partially neutralized with zinc and the Surlyn 8940 is partiallyneutralized with sodium. Sodium carboxylate compounds are ineffective atactivating the cure.

Typically, a sulfur-cured rubber compound will contain 1.7-2.3%elemental zinc (2.1-2.9% ZnO). As shown in the last row of Table 1, theabove compounds with Surlyn 9910 contain 0.04-0.39% elemental zinc(based on 2-20PHR of Surlyn, respectively). At the lowest levels, thisequates to a greater than 97% reduction in elemental zinc.

In zinc stearate activated rubbers with silica fillers, when the amountof Zn+2 relative to carboxyl groups (—COO—I) approaches a stoichiometricratio or when there is a deficit of Zn+2, then a —Si—O—Zn—COO—R group isobserved on the surface of the filler. For the moldable compositionsdescribed herein on the other hand, even though there is a deficit ofZn+2 to carboxyl groups in the ionomers, the reaction on the silicasurface is not apparent via FTIR. Although not being bound by theory, itis proposed that the zinc carboxylates within the ionomer are stericallyhindered or insufficiently mobile to react with the silica hydroxyls.This allows one to reduce the amount of zinc to a very low level andstill activate the cure. This theory of steric hindrance agrees with thelonger Ts2 in the ionomer systems when compared to the traditionalZnO/stearic acid activated systems, which should be quite mobile withinthe rubber compound.

Another way to show activation is by calculating the activation energyvia an Arrhenius plot. This can most easily be accomplished by measuringTs2 at different temperatures.

TABLE 2 Example 1G 1H 1I 1J 1K 1L Component PHR PHR PHR PHR PHR PHR NBR20 20 20 20 20 20 NBR-Styrene 5 5 5 5 5 5 BR 75 75 75 75 75 75 Processaid 0.75 0.75 0.75 0.75 0.75 0.75 Silica 50 50 50 50 50 50 Soybean-oil 22 2 2 2 2 BHT 1 1 1 1 1 1 WAX 0.5 0.5 0.5 0.5 0.5 0.5 PEG 2.5 2.5 2.52.5 2.5 2.5 Process aid 1 1 1 1 1 1 ZnO 3.5 Stearic Acid 1 Surlyn 9910 35 Surlyn 9020 3 Surlyn 6320* 3 <<<<<CURATIVE PACKAGE>>>>> TBzTD 0.210.21 0.21 0.21 0.21 0.21 MBTS 0.6 0.6 0.6 0.6 0.6 0.6 Sulfur 2.21 2.212.21 2.21 2.21 2.21 Activation Energy for Ts2 Each (kJ/mol) 115 88 87 8888 120 *Mg⁺² neutralized ionomer

It is apparent from Table 2 that the Ts2 activation energy for astandard ZnO/stearic acid compound (IH) is essentially equivalent tothat for zinc, partially-neutralized ionomer compounds (1I˜IK). If theneutralizing ion is Mg⁺² (1L), then the activation energy is much higherand similar to a compound without the activator (IG). Later, it will beshown that Mg can have a synergistic effect on cure times and activationenergies when combined with Zn.

In addition to partially neutralized ionomers, being added to a rubbercompound, the precursors can be added and the ionomer generated in situ.DuPont, under the tradename Nucrel, sells the polymeric precursors forionomers. Looking at the activation energies at the bottom of Table 3,it is again apparent that Zn must be present, since in the compoundcontaining un-neutralized Nucrel (1M) the activation energy is similarto compounds without Zn (compare to Compound IG, above).

TABLE 3 Example 1M 1N* IO** Component PHR PHR PHR NBR 20 20 20NBR-Styrene 5 5 5 BR 75 75 75 Process aid 0.75 0.75 0.75 Silica 50 50 50Soybean oil 2 2 2 BHT 1 1 1 WAX 0.5 0.5 0.5 PEG 2.5 2.5 2.5 Process aid1 1 1 ZnO 0.5 0.5 Nucrel 0925 3 3 3 <<<<<CURATIVE PACKAGE>>>>> TBzTD0.21 0.21 0.21 MBTS 0.6 0.6 0.6 Sulfur 2.21 2.21 2.21 Activation Energyfor Ts2 Eact (kJ/mol) 119 88 88 N* - ZnO added on open mill O** - ZnOadded in Banbury

The ZnO was added in different phases of the compounding. For compoundIN, ZnO was added in the last mixing phase, which proceeds at lowertemperature. For Compound 10, ZnO was added in the early mixing phase,which proceeds at higher temperature. For Compound 10, it would beexpected that the earlier addition would provide a longer time and morefavorable (higher) temperatures for reaction between the Nucrel and ZnO.This should lead to better properties in the cured Compound 10 whencompared to Compound IN. Zinc is only 0.24% of the compound weight,which yields a >85% reduction from the typical compound containing1.7-2.3% zinc.

Table 4 below shows that the modulus is highest for Compound 10, thusshowing superior network development. Also, tensile strength andelongation both increase concurrently with the change in order ofaddition. This is further indication of superior network formation inCompound 10, since these parameters normally move conversely to eachother; i.e. as tensile strength increases, % elongation decreases andvice versa.

TABLE 4 Example 1M 1N 1O 300% mod (kg/cm²) 41 49 58 Tensile (kg/cm²) 153163 173 % elongation 509 541 583 Tear (kg/cm) 95 98 99 Abrasion(cc/loss) 0.108 0.090 0.083

Another product is the combination of a partially neutralized zincionomer with further addition of ZnO. In keeping with the goal to reduceoverall Zn contained in the rubber compound, the ZnO is typically addedat much lower levels than normally encountered in rubber processing;however, the lower ZnO amount is not a limitation with respect to thecurrent invention.

It was mentioned previously that Mg could have a synergistic effect whencombined with Zn. It was shown in example 1L that an Mg⁺² neutralizedionomer did not appear to activate the cure. However, if MgO is includedwith an ionomer precursor and ZnO (compare 1P and 1 Q in Table 5), or ifMgO is added to a Zn+2 neutralized ionomer (compare Example 1R and IS inTable 5), then it has a synergistic effect and lowers the activationenergy by 4-5 kJ/mol.

TABLE 5 Example 1P 1Q 1R 1S Component PHR PHR PHR PHR BR 75 75NBR-Styrene 5.9 5.9 5 5 BR 89.1 89.1 ENR-50 15 15 Process aid 1.5 1.51.5 1.5 ZnO 0.5 0.5 Black 2 2 Silica 50 50 45 45 Soybean oil 2 2 1.5 1.5BHT 1 1 1 1 WAX 0.5 0.5 0.5 0.5 PEG 2.5 2.5 2.5 2.5 Process aid 1 1 1 1Surlyn 9910 5 5 Nucrel 0925 5 5 MgO 0.5 0.5 <<<<<CURATIVE PACKAGE>>>>>MBTS 0.6 0.6 0.6 0.6 TBZTD 0.21 0.21 0.21 0.21 Sulfur 2.21 2.21 2.212.21 Activation Energy for Ts2 Eact (kJ/mol) 91 87 97 92

Activation of cure is essential to efficient processing; however,over-activation can lead to poor shelf stability. The above compoundsare extremely stable at room temperature as evidenced by their scorch(Ts2) and cure times (T90) plotted over several days in FIG. 2, with thevalues of Ts2 and T90 remaining constant. In compounds lackingstability, Ts2 and T90 will decrease after several days rendering thecompound too crosslinked to be processed. From Table 5 and FIG. 2, it isalso apparent that the addition of MgO made the compound cure slightlyfaster, but the overall shelf stability was maintained.

Example 2 Maleic Anhydride Carboxylic Polymer Activators

In this example, the use of a metal oxide and a fatty acid to activatesulfur cure in rubber compounds has been replaced by the use of a metaloxide or metal oxides and MA (maleic anhydride) containing polymers. Ina preferred embodiment, the MA containing polymer is based on aterpolymer of (ethylene-acrylic ester-maleic anhydride). These arecommercially available under the tradename Lotader® from Arkema. Inanother preferred embodiment, the MA containing polymer is based on amaleic anhydride grafted (ethyleneacrylate) copolymer. These arecommercially available under the tradename Orevac® from Arkema. Othersuppliers exist for these and similar compounds. Generally, any polymercontaining maleic anhydride is potentially useful in the presentinvention.

TABLE 6 Example 2-1 2-2 2-3 2-4 2-5 2-6 Component PHR PHR PHR PHR PHRPHR NBR 20 20 20 20 20 20 NBR - Styrene 5 5 5 5 5 5 BR 75 75 75 75 75 75Process aid 1.5 1.5 1.5 1.5 1.5 1.5 Silica 50 50 50 50 50 50 Soybean oil2 2 2 2 2 2 Stearic Acid 1 BHT 1 1 1 1 1 1 Wax 0.5 0.5 0.5 0.5 0.5 0.5PEG 2.5 2.5 2.5 2.5 2.5 2.5 Process Aid 1 1 1 1 1 1 Lotader 3 3 3 3 3ZnO 3.5 0.5 0.25 0.25 MgO 0.5 0.5 <<<Curative Package>>> MBTS 0.6 0.60.6 0.6 0.6 0.6 TBzTD 0.21 0.21 0.21 0.21 0.21 0.21 Sulfur 2.21 2.212.21 2.21 2.21 2.21Table 6 gives the formulations of examples 2-1 through 2-6. The mixingprocedure and Rheometer evaluations are carried out as in Example 1.

If the sample is insufficiently activated, then the cure time will belonger. Table 7 contains the cure times (Ts2 and T90) and the physicalproperties for the above examples. A typical formulation is representedby Example 2-1. It contains 3.5 PHR of ZnO and 1 PHR of stearic acid. InExample 2-2, an MA containing polymer, Lotader®, facilitates asignificant reduction in ZnO to 0.5 PHR, with a concomitant improvementin cure time; i.e. T90 is slightly shorter. The physical properties arealso acceptable. If the ZnO is reduced to 0.25 PHR, as in Example 2-3,the cure time is longer and the physical properties, though acceptable,are generally less. These results suggest a less developed networkstructure. If ZnO is totally eliminated from the formula, Example 2-4,then T90 is about two times longer and the Rheometer curve shows a“marching modulus”. This sample was not evaluated further.

To Example 2-3, 0.5 PHR of MgO was added (the result is Example 2-5).These two formulas have similar cure and properties, thus, in this case,there is minimal synergism with MgO. Example 2-6 shows a formula withLotader® and MgO (no ZnO). Its behavior is similar to Lotader® withoutany metal oxide (Example 2-4), showing that Mg⁺² without zinc does notappear to provide any benefits with respect to activation. These resultsparallel those obtained previously with ionomers that were partiallyneutralized with Mg⁺².

TABLE 7 Example 2-1 2-2 2-3 2-4 2-5 2-6 T90 (mins) 3.82 3.40 4.27 8.104.35 8.12 Ts2 (mins) 1.77 2.00 2.65 3.38 2.72 3.50 Hardness (Shore A)72.0 69.5 69.5 68.5 300% Modulus (kg/ 44 50 41 41 cm²) Tensile (kg/cm²)172 141 119 126 % Elongation 653 581 632 658 Tear (kg/cm) 41 64 81 83Abrasion (cc/loss) 0.071 0.051 0.139 0.114Typically, a sulfur-cured rubber compound will contain 1.7˜2.3%elemental zinc. The above compounds with a MA containing polymer have0.12˜0.24% elemental zinc. At the lowest levels, this equates to >90%reduction in zinc.

In zinc stearate activated rubber compositions with silica filler, whenthe amount of Zn⁺² relative to carboxyl groups (—C0₂ ⁻¹) approaches astoichiometric ratio or there is a deficit of Zn⁺², then a˜Si—O—Zn—COO—R group is present on the surface of the silica (where “R”nominally represents a long saturated or partially unsaturated alkylchain). In the activated compositions of Example 2, on the other hand,even though there is a deficit of Zn⁺² to carboxyl groups in the abovecompounds, the reaction on the silica surface is not observed in theFTIR. Not being bound by theory, it is proposed that the maleicanhydride ring opens to form two carboxylic acids. These react with ZnOto form ionic clusters, similar to ionomers. Evidence of the ringopening and ionic clusters can be found in the FTIR. A 1:1 copolymer ofstyrene and maleic anhydride has two easily resolved peaks in the FTIRdue to asymmetric (1850 cm⁻¹) and symmetric (1770 cm⁻¹) stretching of—C0₂ ⁻¹ in the strained anhydride ring. The second peak is shifted fromthe normal symmetric stretching at 1735 cm⁻¹. Once the reaction withzinc oxide occurs in the cured rubber, the two peaks noted above for theanhydride are not present and a peak at 1735 cm⁻¹ appears. Previously,it was noted that a peak at ca. 1469 cm⁻¹ was associated with the zincionomers, both neat product and compounded rubber where a zinc ionomerwas added or it was formed in situ. This peak is apparent in the curedcompounds with maleic anhydride containing polymers, indicating in situformation of a similar structure to zinc ionomers, most likely ionicclusters.

Another way to show activation is by calculating the activation energyvia an Arrhenius plot. This can most easily be accomplished by measuringTs2 at different temperatures. For comparison, a typical ZnO/stearicacid activated system has an E_(act) of about 88 kJ/mol.

In Table 8, the activation energy for several compounds is presented.These results are a significant reduction in activation energy and havenoticeably faster cure times; i.e. T90 is reduced. These compounds havethe added advantage that a lower temperature could be used to effect thecure, thus saving energy. Alternately, time can be saved at the standardcure temperature.

TABLE 8 Example 2-7 2-8 2-9 2-10 2-11 2-12 Component PHR PHR PHR PHR PHRPHR NBR 10 10 10 10 10 10 BR 80 80 80 80 80 80 NR 10 10 10 IR 10 10 10Process aid 2 2 2 2 2 2 Silica 48 48 48 52 52 52 Soybean oil 1 1 1 1 1 1BHT 1 1 1 1 1 1 Wax 0.5 0.5 0.5 0.5 0.5 0.5 PEG 3 3 3 3.5 3.5 3.5Process aid 3 3 3 3 3 3 Orevac 2 Lotader 2 2 2 1 1 ZnO 0.25 0.25 0.250.25 0.25 0.25 MgO 0.25 0.5 0.5 <<< Curative Package >>> MBTS 0.6 0.60.6 0.6 0.6 0.6 TBzTD 0.18 0.18 0.18 0.18 0.18 0.18 Sulfur 1.8 1.8 1.82.06 2.06 1.8 <<< Rheometer data >>> T90 (mins) 2.48 2.62 2.63 2.47 2.552.37 Ts2 (mins) 1.65 1.65 1.67 1.47 1.53 1.45 E_(act) (kJ/mol) 78.1 76.475.7 77.2 76.7 73.4 <<< Physical properties >>> Hardness 67.5 67.5 64.567.5 67.5 70.5 (Shore A) 300% mod. 33 35 37 35 35 41 (kg/cm²) Tensile(kg/ 114 116 109 114 117 139 cm²) % Elongation 706 690 621 695 706 818Tear (kg/cm) 68 73 68 74 73 75 Abrasion (cc/ 0.337 0.257 0.321 0.1950.171 0.141 loss)

Another useful type of compound is a copolymer of styrene and maleicanhydride. These can be used with low levels of zinc to produce curedrubber components. Like the other maleic anhydride containing polymers,the activation energy is very low. Examples 2-13 through 2-20 in table 9are illustrative of these copolymers. SMA 1000 is a low molecular weightstyrene-maleic anhydride copolymer with an approximately 1:1 mole ratio.SMA 3000 is a low molecular weight styrene-maleic anhydride copolymerwith an approximately 3:1 mole ratio of styrene to maleic anhydride. SMA3840 is a partial monoester of a styrene-maleic anhydride copolymer. SMAEF80 is a low molecular weight styrene-maleic anhydride copolymer withan approximately 8:1 mole ratio of styrene to maleic anhydride. The SMAresins are commercial products sold by Sartomer Company of Exton Pa.

TABLE 9 Example 2-13 2-14 2-15 2-16 2-17 2-18 2-19 2-20 Component PHRPHR PHR PHR PHR PHR PHR PHR NBR 10 10 10 10 10 10 10 10 BR 80 80 80 8080 80 80 80 NR 10 10 10 10 10 10 10 10 Process aid 2 2 2 2 2 2 2 2Silica 48 48 48 48 48 48 48 48 Soybean oil 1 1 1 1 1 1 1 1 BHT 1 1 1 1 11 1 1 Wax 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 PEG 3 3 3 3 3 3 3 3 Processaid 3 3 3 3 3 3 3 3 SMA1000 2 2 SMA3000 2 2 SMA3840 2 2 SMA EF80 2 2 ZnO0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 <<<Curative Package>>> MPTS 0.60.6 0.6 0.6 Sulfur donor* 0.6 0.6 0.6 0.6 TBzTD 0.18 0.18 0.18 0.18 0.180.18 0.18 0.18 Sulfur 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 *diisopropylxanthogen polysulphideThe mixing procedure and Rheometer evaluations are as previouslydescribed. Physical and chemical properties are given in Table 10.

TABLE 10 Example 2-13 2-14 2-15 2-16 2-17 2-18 2-19 2-20E_(act (kJ/mol)) 890 75.5 77.6 73.2 83.7 77.5 80.4 73.4 Hardness (ShoreA) 67.5 67.5 69 69 67.5 67.5 64.5 67.5 300% mod. (kg/cm²) 35 35 39 39 3433 27 30 Tensile (kg/cm²) 116 108 123 120 109 105 96 110 % Elongation730 695 738 708 732 715 761 748 Tear (kg/cm) 72 63 74 69 74 65 71 73Abrasion (cc/loss) 0.158 0.172 0.13 0.133 0.229 0.23 0.324 0.26The compounds have generally acceptable properties, but the ones with ahigher concentration of maleic anhydride to styrene tend to give betterproperties. (compare 2-13, a 1:1 ratio to 2-19 a ⅛:1 ratio).

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention. Individual elements or features ofa particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the invention, and all such modificationsare intended to be included within the scope of the invention.

1. A curable rubber composition comprising: at least one rubber that canbe crosslinked by a sulfur cure, a sulfur-based rubber curing agent; anactivator component comprising a copolymer containing a plurality ofCOOH groups, wherein at least a portion of the COOH groups isneutralized with at least zinc and wherein the curable rubbercomposition comprises 0.01-0.5% by weight Zn.
 2. A curable rubbercomposition according to claim 1, wherein the copolymer is an α-olefincopolymer.
 3. A curable rubber composition according to claim 1, whereinthe copolymer is a copolymer of an α-olefin RCH═CH₂, wherein R ishydrogen or an alkyl radical having from 1 to 8 carbon atoms, and anα,β-ethylenically unsaturated carboxylic acid having 1 or 2 carboxylicacid groups.
 4. A curable rubber composition according to claim 3,wherein the α-olefin content of the copolymer is at least 50 mol %.
 5. Acurable rubber composition according to claim 3, wherein theα,β-ethylenically unsaturated carboxylic acid content of the copolymeris 0.2 to 25 mol %.
 6. A curable rubber composition according to claim3, wherein at least 10% of the COOH groups is neutralized with zinc. 7.A curable rubber composition according to claim 3, wherein theα,β-ethylenically unsaturated carboxylic acid is a member selected fromthe group consisting of acrylic acid, methacrylic acid, and combinationsthereof.
 8. A curable rubber composition according to claim 7, whereinthe α-olefin is ethylene.
 9. A curable rubber composition according toclaim 3, wherein the copolymer is a copolymer of the α-olefin, theα,β-ethylenically unsaturated carboxylic acid, and another monomer. 10.A curable rubber composition according to claim 1, wherein the copolymercomprises a copolymer selected from the group consisting ofethylene/acrylic acid copolymers, ethylene/methacrylic acid copolymers,ethylene/methyl hydrogen maleate copolymers, ethylene/maleic acidcopolymers, ethylene/acrylic acid/methyl methacrylate copolymers,ethylene/methacrylic acid/ethyl acrylate copolymers, ethylene/methylhydrogen maleate/ethyl acrylate copolymers, ethylene/methacrylicacid/vinyl acetate copolymers, ethylene/acrylic acid/vinyl alcoholcopolymers, ethylene/styrene/acrylic acid copolymers, and styrene/maleicanhydride copolymers and monoesters and partial esters thereof.
 11. Acurable rubber composition comprising: at least one rubber that can becrosslinked by a sulfur cure, a sulfur-based rubber curing agent; anactivator component comprising a copolymer containing a plurality ofCOOH groups, wherein at least a portion of the COOH groups isneutralized with at least zinc and with a member selected from the groupconsisting of calcium, magnesium, and combinations thereof.
 12. Acurable rubber composition according to claim 11, wherein the moldablerubber composition comprises 0.01-0.5% by weight Zn.
 13. A curablerubber composition according to claim 11, further comprising anaccelerator.
 14. A curable rubber composition according to claim 11,wherein the copolymer is an α-olefin copolymer.
 15. A curable rubbercomposition according to claim 11, wherein the copolymer is a copolymerof an α-olefin RCH═CH₂, wherein R is hydrogen or an alkyl radical havingfrom 1 to 8 carbon atoms, and an α,β-ethylenically unsaturatedcarboxylic acid having 1 or 2 carboxylic acid groups.
 16. A curablerubber composition according to claim 15, wherein the α-olefin contentof the copolymer is at least 50 mol %.
 17. A curable rubber compositionaccording to claim 15, wherein the α,β-ethylenically unsaturatedcarboxylic acid content of the copolymer is 0.2 to 25 mol %.
 18. Acurable rubber composition according to claim 15, wherein at least 10%of the COOH groups is neutralized with zinc.
 19. A curable rubbercomposition according to claim 15, wherein the α,β-ethylenicallyunsaturated carboxylic acid is a member selected from the groupconsisting of acrylic acid, methacrylic acid, and combinations thereof.20. A curable rubber composition according to claim 19, wherein theα-olefin is ethylene.
 21. A curable rubber composition according toclaim 15, wherein the copolymer is a copolymer of the α-olefin, theα,β-ethylenically unsaturated carboxylic acid, and another monomer. 22.A curable rubber composition according to claim 11, wherein thecopolymer comprises a copolymer selected from the group consisting ofethylene/acrylic acid copolymers, ethylene/methacrylic acid copolymers,ethylene/methyl hydrogen maleate copolymers, ethylene/maleic acidcopolymers, ethylene/acrylic acid/methyl methacrylate copolymers,ethylene/methacrylic acid/ethyl acrylate copolymers, ethylene/methylhydrogen maleate/ethyl acrylate copolymers, ethylene/methacrylicacid/vinyl acetate copolymers, ethylene/acrylic acid/vinyl alcoholcopolymers, ethylene/styrene/acrylic acid copolymers, and styrene/maleicanhydride copolymers and monoesters and partial esters thereof.
 23. Acured rubber composition prepared by curing the curable rubbercomposition of claim
 1. 24. A cured rubber composition prepared bycuring the curable rubber composition of claim
 11. 25. A method ofcuring a rubber composition, comprising: including in the rubbercomposition a sulfur-based rubber curing agent and an activatorcomponent comprising a copolymer containing a plurality of COOH groups,wherein at least a portion of the COOH groups is neutralized with atleast zinc and wherein the moldable rubber composition comprises0.01-0.5% by weight Zn or wherein the at least a portion of the COOHgroups is neutralized also with a member selected from the groupconsisting of calcium, magnesium, and combinations thereof and curingthe rubber composition.
 26. A method of curing a rubber compositionaccording to claim 25, wherein the copolymer is an α-olefin copolymer.27. A method of curing a rubber composition according to claim 25,wherein the copolymer comprises a copolymer selected from the groupconsisting of ethylene/acrylic acid copolymers, ethylene/methacrylicacid copolymers, ethylene/methyl hydrogen maleate copolymers,ethylene/maleic acid copolymers, ethylene/acrylic acid/methylmethacrylate copolymers, ethylene/methacrylic acid/ethyl acrylatecopolymers, ethylene/methyl hydrogen maleate/ethyl acrylate copolymers,ethylene/methacrylic acid/vinyl acetate copolymers, ethylene/acrylicacid/vinyl alcohol copolymers, ethylene/styrene/acrylic acid copolymers,and styrene/maleic anhydride copolymers and monoesters and partialesters thereof.
 28. A cured rubber composition prepared by the method ofclaim 25.